This invention is related delivery of laser or other source of thermal energy to biological or other tissue for treatment therein, and more particularly, to a method and system for delivery of the laser or other source of thermal energy to the target tissue wherein surrounding tissue, including surface tissue, is also elevated in temperature by conduction of heat from the target tissue, and wherein thermal quenching of the surrounding tissue, and in particular the surface tissue, prevents thermal damage thereto.
It is sometimes desirable to cause heat affected changes in a selected structure in tissue, such as a vein or hair follicle, without causing heat affected changes in tissue adjacent to the selected structure. The prior art treatments use a method called selective photothermalysis, whereby laser or pulsed light source is tuned to a wavelength whereby its energy is preferentially absorbed by a preselected target. The energy from the source is delivered within a time period short enough for heat to build up in the target and faster than it flows into adjacent regions by thermal conduction. The amount of energy or fluence delivered to the target is chosen such that the temperature rise in the targeted region results in an intended thermal treatment of the target.
Vascular lesions have been treated for more than twenty years with a variety of lasers and light sources including pulsed dye lasers, argon lasers, Nd:YAG lasers, and flashlamps. The pulsed dye laser operating at a wavelength of 577 nanometers is very effective since it can penetrate through skin and is S absorbed by hemoglobin in smell veins resulting in heat build up and a photo-coagulation of the vein. The energy is confined to a short time period, less than the thermal relaxation time of the vessel being treated, so that heat loss to surrounding tissue is minimized during treatment. The principal is known as, or at least has been characterized as, selective photothermalysis. Selective Photothermalysis, Anderson, R. R, Parrish, J. A., Science 1983 Vol 220 Pages 524-527
Although the pulsed dye laser is useful for many smaller vessels, in lesions such as port wine stains, the larger and deeper lying vessels found in leg telangiactasias and other undesirable lesions are difficult to treat. The pulsed dye laser energy is absorbed too strongly by hemogolobin and so does not penetrate fully though larger veins which approach diameters of 0.1 mm to 3 mm in diameter. Larger vessels also require more energy to achieve the same coagulative effect and have longer thermal relaxation times. A variety of lasers and a non-coherent intense light source with tunable wavelength have all been used to treat vessels of different sizes and depths in skin.
Melanin absorption of laser energy results in some heating of the epidermis by each of the various energy sources used for vascular treatment. Several methods have been described for cooling the surface of skin during treatment to minimize the risk of thermal injury to tissue adjacent to the targeted veins. One early method included pre-cooling with ice for several minute prior to treatment.
U.S. Pat. No. 5,282,797 issued Feb. 1, 1994 to Chess describes a method of circulating cooling fluid over a transparent plate in contact with the treatment area to cool the epidermis during treatment.
U.S. Pat. No. 5,344,418 issued Sep. 6, 1994 to Ghaffari describes a method whereby a coolant is used for a predetermined time interval in coordination with the delivery of laser energy to optimize the cooling of the epidermis and minimize cooling of the targeted vessel.
U.S. Pat. No. 5,814,040 issued Sep. 29, 1998 to Nelson et al. describes a dynamic cooling method whereby a cryogenic spurt is applied for a predetermined short time directly onto the skin in the target region. The time period is well controlled and limited so that cooling is confined only to the epidermis while leaving the temperature of deeper port wine stains substantially unchanged.
The result of the various cooling methods is that a greater fluence can be used to treat vessels without significant thermal damage during treatment to the epidermis. Avoiding epidermal damage is extremely important for the treatment of deeper and larger vessels since the fluences and wavelengths used could cause substantial damage to uncooled epidermis.
Problems associated with the prior art include the subsequent conduction of heat away from the treated vessels or other target tissue into adjacent tissue. For larger vessels, a significant amount of heat builds up during the treatment. The treated vessels cool off by thermal conduction to surrounding tissue. The temperature of the tissue adjacent to the vessel will rise immediately after treatment and may reach levels causing significant patient discomfort and even epidermal damage.
Therefore what is needed is a method and device which subsequently cools and quenches heat build up in tissue, and especially in surface tissue, adjacent to tissue or structures treated in the thermally-mediated process or treatment.
It is therefore an advantage and an object of the present invention to provide an improved system for selectively cooling tissue during photothermal treatment.
It is a further advantage of the present invention to provide such a system which uses dynamic cooling to quench heat build up during and after photothermal treatment.
It is a further advantage of the present invention to provide such a system which selectively heats a subsurface structure in tissue and subsequently quenches heat build up in non-target tissue.
It is a further advantage of the present invention to reduce the level of pulsed energy needed for treatment by minimizing precooling of the tissue.
It is a further advantage of the present invention to provide such a system which selectively heats a subsurface structure in skin to cause thermal affected changes in said subsurface structure without significant epithelial damage due to subsequent heating from the target region.
It is a further advantage of the present invention to provide such a system which selectively heats vascular lesions in tissue and quenches subsequent heat build up in epithelial tissue.
It is a further advantage of the present invention to provide such a system which selectively heats hair follicles in tissue and quenches subsequent heat build up in epithelial tissue.
It is a further advantage of the present invention to require less cooling of the target area than is typically required, resulting in more efficient heating of the selected target and less thermal damage to surrounding tissue.
In a preferred embodiment, the system for generating light energy is a laser system such as but not limited to a solid-state laser, including but not limited to a neodymium-doped yttrium-aluminum-garnet (Nd:YAG) laser.
In additional preferred embodiments, the system for generating light energy is a gas discharge flashlamps or an incandescent-type filament lamp.
The energy from the generating system may be directed into or coupled to a delivery device such as but not limited to a fiber optic or articulated arm for transmitting the light energy to the target tissue.
The light energy may be focused on tissue with a focusing lens or system of lenses.
The surface of the tissue may be cooled with a cooling device including but not limited to an irrigating solution, a spray or flow of refrigerant or other cryogenic material, or a transparent window cooled by other active means, or other dynamic or passive cooling means.
The tissue may be preheated with a heating device such as, but not limited to an intense light source, a flashlamp, a filament lamp, laser diode, other laser source, electrical current, or other electromagnetic or mechanical energy which penetrates into layers of tissue beneath the surface. The preheating can occur simultaneously or just prior to the surface cooling of tissue from the cooling device such that the tissue preheating results in a temperature rise in underlying layers of tissue, and a temperature profile results. The pulsed application of energy from the energy delivery device results in a temperature profile that preferentially heats a selected structure or target in tissue, and the post cooling prevents thermal damage to tissue adjacent to that structure. This also reduces the overall pulse energy level needed of the pulsed treatment device due to the fact that a desirable temperature profile exists prior to delivery of the pulsed treatment energy.
The tissue may be post cooled with a dynamic cooling device such as, but not limited to a pulse, spray or other flow of refrigerant such that the post cooling occurs after a temperature rise in an underlying targeted structure and a temperature profile results such that the pulsed application of energy from the energy delivery device results in a temperature profile that preferential heats a selected structure in tissue without subsequent undesirable heating to tissue adjacent to that structure from thermal conduction.